The present invention relates to global positioning system receivers and more particularly to continuous and sequential tracking global positioning system receivers.
The global positioning system includes a constellation of satellites transmitting navigation information via radio signals. Time and position may be calculated by receivers which receive and process these radio signals. The constellation of GPS satellites broadcast two binary phase shift keying (BPSK) modulated signals at L-band.
The first, L1, is at 1575.42 megaHertz and the second, L2, is at 1227.6 megaHertz. The modulated signals include pseudorandom noise (PN) codes and data. The L1 signal carrier is modulated in quadrature with both a clear acquisition, or CA, code, and a precise, or P, code. The CA code chipping rate is 1.023 megaHertz and that of the P code is 10.23 megaHertz. The L2 frequency is normally modulated with only the P code. The modulation on the L1 and L2 signals is contained in a frame which is 1500 bits long and having thirty seconds duration.
It is necessary to track at least four GPS satellites in order to compute the three dimensional position of a GPS receiver and the time. One method, termed continuous tracking, is to dedicate a receiver channel to track a given satellite. A group of four or more such channels operate in parallel, providing data from at least four satellites, allowing unique determination of positional data.
Another method of tracking four satellites with fewer than four reception channels requires time-sharing receiver channels among the satellite signals. Two primary types of time multiplexing arrangements are common, known as satellite multiplex and satellite sequencing receivers.
Satellite multiplex receivers typically use a single hardware channel which is switched between four or more satellite signals, providing a very simple receiver architecture. These signals are multiplexed at a rate such that signals from all observed satellites are observed in a single satellite signal data bit time of 20 milliseconds. In this type of arrangement, multiplexing rates are faster than the loop bandwidth. A reduction in signal-to-noise ratio occurs because each satellite is observed for less than the receiver loop settling time.
Satellite sequencing receivers slowly switch between multiple satellite signals at a rate substantially slower than the loop bandwidth but which does not allow decoding of the 50 BPS data message. No reduction in signal to noise ratio is incurred at this slower switching rate, providing a clear advantage without incurring any sacrifice in the form of additional hardware complexity.
Sequencing and multiplexing receivers are thus forced to adopt bimodal operation wherein range measurements and data collection are performed at separate times. Multiple ranging measurements needed to compute position are not possible during the data collection interval. Both sequencing and multiplex receivers can operate with as few as a single channel, but without providing continuous monitoring of either positional data or satellite data.
Data loss may result during lockup or synchronization to a particular satellite signal in the course of the sequencing process. Accordingly, such operations require receiver circuitry which may be set up rapidly by control circuitry when time-sharing operations cause switching from one satellite signal to another. In addition, high Doppler frequencies require high-speed logic to process information more rapidly.
In order to produce low-cost, flexible GPS receivers, sequencing mode operation using few receiver channels is desired. Prior art receivers operate in the continuous tracking mode, requiring four or more channels, or in the sequencing or multiplexing modes, with attendant sacrifices in current satellite data and signal-to-noise ratio. Prior art GPS receivers do not provide needed flexibility and hence desired low-cost features.
Another shortcoming of the prior art is the inability to provide a low-cost integrated circuit chip set embodying circuitry of the requisite speed to handle a number of channels for sequencing or continuous tracking mode operations. Typically such circuitry requires several integrated circuits and discrete components matched for signal delay and phase shift.
Accordingly, it is extremely desirable to be able to provide multi-channel GPS receivers, having as few channels as possible, having high operating and switching speed, using as few parts as possible to provide the highest signal to noise ratio achievable, and at low cost.